A liquid crystal display generally comprises a liquid crystal cell, a polarizing element and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing elements are provided on both sides of the liquid crystal cell, and one or two optical compensatory sheets are placed between the liquid crystal cell and the polarizing element. A display of reflection type comprises a reflection plate, a liquid crystal cell, an optical compensatory sheet and a polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. The liquid crystal cells can be classified into various display modes according to alignment of the rod-like liquid crystal molecules in the cell. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include TN mode and HAN (hybrid aligned nematic) mode.
The optical compensatory sheet is widely used in various liquid crystal displays because it prevents the displayed image from undesirable coloring and enlarges a viewing angle of a liquid crystal cell. A stretched polymer film has been used as the optical compensatory sheet.
Japanese Patent Provisional Publication No. 2(1990)-264905 discloses a process for preparing an optical compensatory sheet. In the process, a polymer that forms, by stretching, a refractive anisotropic body showing positive birefringence is biaxially stretched. The thus-prepared optically biaxial polymer film is particularly effective in optically compensating a liquid crystal cell containing rod-like liquid crystal molecules oriented in homeotropic alignment (e.g., a cell of VA mode).
An optical compensatory sheet can be formed from liquid crystal molecules in place of using the stretched polymer film. An optically anisotropic layer containing the liquid crystal molecules is provided on a transparent support. Since the liquid crystal molecules have various alignment forms, an optical compensatory sheet obtained by using the liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched polymer film.
Rod-like or discotic liquid crystal molecules are used in the optical compensatory sheet. A rod-like liquid crystal molecule is positively optically uniaxial. In detail, two principal refractive indices (indices along two directions perpendicular to the long axis of the rod-like molecule) are essentially identical, and the other principal refractive index (along the direction parallel to the long axis of the molecule) is larger than the two indices. In contrast, a discotic liquid crystal molecule is negatively optically uniaxial. In detail, two principal refractive indices (indices along two directions in a discotic plane of the molecule) are essentially the same, and the other principal refractive index (along the direction parallel to normal of the discotic plane) is smaller than the two indices.